Synthesis, Mesomorphism, Photophysics, and Device Properties of Liquid-Crystalline Pincer Complexes of Gold(III) Containing Semiperfluorinated Chains

Gold(III) complexes of C∧N∧C-coordinating 2,6-diphenylpyridine pincer ligands with arylacetylide co-ligands are known triplet emitters at room temperature. We have reported previously that by functionalizing both the pincer ligand and the phenylacetylene with alkoxy chains, liquid crystallinity may be induced, with the complexes showing columnar mesophases. We now report new derivatives in which the phenylacetylene incorporates one, two, or three 1H,1H,2H,2H-perfluoroalkyl chains. In terms of intermolecular interactions, solution 1H NMR experiments suggest that the semiperfluoroalkyl chains promote a parallel, head-to-head arrangement of neighboring molecules relative to one another, rather than the anti-parallel, head-to-tail orientation found for the all-hydrocarbon materials. In terms of the liquid crystal properties, the complexes show columnar phases, with the addition of the more rigid fluorocarbon chains leading to a stabilization of both the crystal and liquid crystal mesophases. Mesophase temperature ranges were also wider. Interestingly, the amphiphilic nature of these complexes is evident through the observation of a frustrated columnar nematic phase between a Colr and a Colh phase, an observation recently reported in detail for one compound (Liq. Cryst., 2022, doi: 10.1080/02678292.2021.1991017). While calculation shows that, despite the “electronic insulation” provided by the dimethylene spacer group in the semiperfluoroalkyl chains, a small hypsochromic shift in one component of the absorption band is anticipated, experimentally this effect is not observed in the overall absorption envelope. Complexes with substituents in the 3,3′,4,4′-positions of the phenyl rings of the pincer ligand once more show higher-luminescence quantum yields than the analogues with substituents in the 4,4′-positions only, associated with the lower-energy-emissive state in the former. However, in contrast to the observations with all-hydrocarbon analogues, the luminescence quantum yield of the complexes with 3,3′,4,4′-substitution on the pincer increases as the number of semiperfluoroalkyl chains on the phenylacetylide increases, from 20% (one chain) to 34% (three chains). External quantum efficiencies in fabricated OLED devices are, however, low, attributed to the poor dispersion in the host materials on account of the fluorinated chains.


Instrumentation
H NMR spectra were measured on a Jeol ECS400 spectrometer operating at 400 MHz with chemical shifts referred to residual non-deuterated CHCl3 signals. Concentration-dependent 1 H spectra were measured on a Bruker 500 AVANCE II spectrometer operating at 500 MHz.
Mass spectra (ESI and APCI) were collected on Bruker compact time of flight mass spectrometer; spectra were internally calibrated using sodium formate as the calibrant. Samples were transferred to the spectrometer an Agilent 1260 Infinity LC system. used as an internal reference. Cyclic voltammetry was performed between +0.7 and -2.5 V for 3 scans at a scan rate of (100 mV s -1 ).
Elemental analysis was carried out using an Exeter Analytical Inc. CE-440 Analyser and Sartorius S2 analytical balance; calibration was performed against acetanilide standards and checked by the use of S-benzyl thiouronium chloride as internal standard. Obtaining good CHN data for materials containing perfluorocarbon units is often rather challenging and for complex 19a we found an error in carbon of 0.9%, while in a couple of other complexes the error was ca 0.7%.
Such complexes were prepared more than once with similar results. However, their thermal transitions were as sharp as those for which the CHN data were within normally acceptable limits and so we believed that it was appropriate to report their properties, none of which appear out of line when compared with close homologues.
Polarising optical microscopy was carried out using an Olympus BX50 polarising microscope equipped with a Linkam scientific LTS350 heating stage, Linkam LNP2 cooling pump, and Linkam TMS92 controller, differential scanning calorimetry was performed on a Mettler DSC822 e using Mettler STAR-E software, which was calibrated before use against indium and zinc standards under an atmosphere of dry nitrogen. Small-angle X-ray scattering was recorded using a Bruker S3 D8 Discover equipped with a temperature controlled, bored graphite rod furnace, custom built at the University of York. Cu-Kα (l = 0.154056 nm) radiation was used, generated from a 1 μS microfocus source. Diffraction patterns were recorded on a 2048 × 2048 pixel Bruker VANTEC 500 area detector set at a distance of 121 mm from the sample, allowing simultaneous collection of small angle and wide angle scattering data. Samples were measured in 1 mm capillary tubes in a magnetic field of ca 1 T.
The absorption spectra of the complexes were measured in solution in CH2Cl2 in 1 cm pathlength quartz cuvettes using a Biotek Instruments XS spectrometer. Emission spectra were recorded using a Jobin Yvon Fluoromax-2 spectrometer equipped with a Hamamatsu R928 photomultiplier tube (PMT). For the measurements at 298 K, the solutions were contained within 1 cm pathlength quartz cuvettes modified for connection to a vacuum line. Degassing was achieved via a minimum of three freeze-pump-thaw cycles whilst connected to the vacuum manifold; final vapour pressure at 77 K was < 5 x 10 -2 mbar, as monitored using a Pirani gauge. Luminescence quantum yields were determined using aqueous [Ru(bipy)3]Cl2 as the standard (f = 0.040 in airequilibrated aqueous solution). Emission spectra at 77 K were recorded in a glass of EPA (= diethyl ether / isopentane / ethanol, 2:2:1 v/v) in 4 mm diameter tubes held within a liquidnitrogen-cooled quartz dewar. The luminescence lifetimes of the complexes in deoxygenated solution and at 77 K were measured by multi-channel scaling following excitation into the lowestenergy absorption band using a microsecond pulsed xenon lamp; an appropriate excitation wavelength corresponding to the low-energy absorption band of the complexes was selected by means of a monochromator. The emitted light was detected at 90˚ using a Peltier-cooled R928 PMT after passage through a monochromator. The lifetimes in air-equilibrated solution (<10 μs) were measured by time-correlated single photon counting (TCSPC), following excitation at 374 nm with a pulsed laser diode.
1H,1H,2H,2Hperfluoroalkan-1-ol and pyridine were added (in varying solvents) and the reaction mixture was stirred under a flow of N2 for 1 hour. The resulting precipitate was removed by S4 filtration and volatiles removed from the filtrate in vacuo. The residue was then purified by flash column chromatography on silica gel.
The resulting reaction mixture was stirred at room temperature for 16 hours, after which the precipitate which formed was isolated by filtration and washed with water and acetonitrile and air dried. The product was used without further purification.

Cyclic Voltammetry
Data were recorded as described previously, 1 although the lower solubility for 21b limited the amount that could be dissolved. The combined voltammograms are shown as Figure S13.  Calculations with a solvent model were performed in Gaussian16 10 using the PBE0 functional and def2-TZVPP basis set with an SCRF solvent model for CH2Cl2.